Black stains on wood ruin the look of a piece of furniture. These stains are usually caused by water, but can also be caused by oil, ink, cigarette burns or mould. Removal techniques range from the simple application of wood bleach to the most difficult--removing the furniture's finish and sanding down the entire surface.
Remove the furniture's gloss finish over the stain with a piece of fine sandpaper. Keep the finish removal to a minimum, only removing the finish over the black stain.
Test the bleach on a small inconspicuous area of the furniture first to make sure it does not bleach the wood. If the bleach does not ruin the colour of the wood, proceed to applying bleach to the stain.
Select a bleach to use on the black stain. Start with the weakest bleach first to see if it works. There are four basic types of bleach that can be used on wood. From weakest to strongest, they are: oxalic acid, chlorine laundry bleach, chlorine pool bleach and peroxide bleach. Oxalic acid is a mild wood bleach that is effective in removing black water and mildew stains on wood. Oxalic acid does not strip the wood's natural colour. Chlorine bleach, used for laundry, works well at removing dye-based stains. Chlorine swimming pool bleach is stronger and can be used for more stubborn stains. Peroxide bleach is a two-part bleach of sodium hydroxide and hydrogen peroxide. Mix them together just before application. This type of bleach may also remove the natural pigment of the wood, so use caution.
Brush the weakest bleach (oxalic acid) on the black stain using a small paintbrush. Use only enough bleach to cover the stain. Let the bleach soak in for a couple of hours. Apply another coat of bleach and let it soak in overnight. Use one of the stronger bleaches if the oxalic acid is not effective.
Sand deeper into the wood, if the stain is still visible. Using the fine sandpaper, remove another layer of wood and sand deeper into the stain. Re-bleach the stain after sanding to see if it lightens it further.
Wipe down the area with one part vinegar to one part water to neutralise the bleach.
Stain the rest of the piece of furniture darker to match the stain if you were not able to remove the black stain.
If your furniture is an antique, consult a professional before attempting to remove the black stain. Changing the finish on an antique can lower its value.
Wear gloves, face mask and eye mask when using bleach or oxalic acid. Wear old clothing when working with bleach.